<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <style>
    @keyframes first{
        from{
            width: 100px;
            height: 100px;
        }
        to{
            width: 200px;
            height: 200px;
        }
    }
    div{
        width:100px;
        height: 100px;
        background: skyblue;
    }
    div:hover{
        animation: first 1s;
        /* animation:keyName time infinite  */
    }
    </style>
</head>
<body>
    <div></div>
</body>
</html>